1. Field of the Invention
The present invention relates to a method of controlling the downhill running of a vehicle and an apparatus therefor. More specifically, the invention relates to technology for improving the shift-down operation control and for improving the engine brake operation control during the downhill running.
2. Related Art of the Invention
In the speed control operation of an automatic transmission of vehicles, there has generally been employed a system for determining a transmission gear ratio (or speed-change gear) by making a reference to a speed-change map in which has been set in with the transmission gear ratios (or speed-change gears) depending upon the opening degree of the throttle valve and the vehicle speed. However, the mechanism for determining the transmission gear ratio (speed-change gear) has been constituted without at all taking into consideration the changes in the running resistance such as changes of the inclination and the like. In running uphill, for example, an unnecessary shift-up operation is executed by separating the foot away from the accelerator pedal at the time when the vehicle enters into a corner. When running downhill, the gear of the highest speed is selected, which no longer makes it possible to apply the engine brake causing the foot brake to bear an increased burden.
Therefore, a variety of methods have heretofore been proposed for changing the speed-change schedule depending upon the inclination (running resistance). According to the method disclosed in, for example, Japanese Unexamined Patent Publication No. 5-231522, the downhill running state is detected based upon a detection signal from an inclination (acceleration) sensor. Besides, when it is detected that the accelerator pedal has not been depressed, the shift-down operation is executed to effectively apply the engine brake during the downhill running.
According to the method disclosed in the above Japanese Unexamined Patent Publication No. 5-231522, however, the shift-down operation is executed by taking into consideration only the downhill inclination and the fully closed state of the throttle valve. In the following cases, therefore, the driver feels the driving uncomfortable and the operation is carried out involving an uncomfortable feeling.
For example, when running a long straight downhill, the driver may wish to continue the coasting using the gear of the highest speed (e.g., fourth speed) without shifting down the gear. With the above-mentioned conventional transmission, however, the shift-down operation is forcibly executed without reflecting the intention of the driver. Therefore, the driver feels uncomfortable. Alternatively, the driver must depress the accelerator pedal if he intends to drive his car as he likes. The driver feels uncomfortable to a considerable degree if he is forced to depress the accelerator pedal in the downhill running. Besides, such an operation interrupts the control for cutting the fuel during the deceleration running, which is detrimental to fuel efficiency and exhaust performance.